GPS with no cell service

This is not my experience. On a recent trip abroad I had data-roaming off (for obvious reasons) and with no access to an open wi-fi network no GPS position is ever found. I use imarkmyspot which does not use maps, just finds your position. But it does not work without a data connection.

Could it be possible that perhaps the app itself is coded in such a way that when opened, it "needs" to detect a data connection in order for it to work correctly? IIRC, there is at least one GPS app that will run w/o first needing to detect a data connection (e.g., WiFi, 3G, EDGE, etc.)...apparently, the one you're using isn't it.

Not in my experience. My App (Twittelator) successfully asked if it could get the location form the iPhone and got the correct lat/long from the device. This was 10 mins after signal from any cell network or wifi was lost and 10 mins before we got signal again. I then "twittered" the post when back in 3G cellnet range.

So I clearly demonstrated to myself that with no phone or wifi network available and outdoors, the iPhone does offer a GPS coordinate to applications that request it.

See the twitpic I took which embedded the link to google maps showing the received lat/long from the iPhone GPS with no Cell coverage, outdoors. (This is a Twittelator function) http://twitpic.com/dd84 and I can confirm the location to be correct.

In contrast, here was a twitter where the application got both a GPS lat/long and had cell net 3G access http://twitpic.com/dcyc

is everyone here using the NEW 3G or have some just been inputting generic iphone responses???

i agree with ice and if you read the technical data you will see the new and old iphones operate differently when it comes to the GPS.

bons... it sounds like you keep describing the old iphone's functionality when it comes to the GPS... which was software driven because there was not a chip on the board.

I'd like to ask a question to elaborate on this topic. I've been in the woods a lot lately and the GPS jumps around quite a bit, and I frequently have the lowest cellular signal possible without actually having "no signal". Thank you icebike for the links regarding the infineon chip in the iphone 3g, could you let me know generally how it compares to other major GPS chipsets? For example, the only other chip I have experience with, the SirfStar III that is in my car GPS unit... how does the iphone's infineon chip compare with that in terms of accuracy and ability to detect weak signals? Do you know how many satellites the iphone's chip tracks at once to determine the signal?

You seem very knowledgeable about this hardware in the iphone, please elaborate a bit more and give me a good comparison between it and the SirfStar setup in my Magellan.

Cheers!

Antenna is a big factor for GPS. A dedicated GPS like Magellan or Garmin has a good size antenna to help maintain a satellite lock. In the woods with a weak cell signal and no clear line of sight to the sky, the GPS on the iPhone is going to have a hard time maintaining a position.

how does the iphone's infineon chip compare with that in terms of accuracy and ability to detect weak signals? Do you know how many satellites the iphone's chip tracks at once to determine the signal?

The chip family that the iphone uses has published the ability to track 1t least 12 birds at once, but its not clear that the actual model in the iphone can do that, nor is it clear if it can track 12 in parallel or by time-slicing. See: http://www.gpsinformation.org/dale/why12.htm The key takeaway is that even three physical channels can give you 12 channel tracking, and only 4 are needed for a fix.

As for comparison to other chip sets, bear in mind that this chip was selected based on cost, and the requirement to perform E-911 functions, and anything else is gravy.

What you get in a hunting GPS or a Garmin are typically much more accurate, down to 12 foot accuracy on my Garmin. Best I've seen on the iPhone is 56 feet.
(Accuracy estimates are reported by some apps like the Track Thing, or gpsCompass (both free in the app store). Both can tell you exactly where you are without any map data from the cellular network.

Accuracy is one thing, Sensitivity is another.

The Infineon chipsets great claim to fame is the low low power and high sensitivity. Some of this alleged sensitivity comes from the fact that it gets its catalog information from cell towers, which means it knows exactly when specific satellites are supposed to be over head and exactly which frequency to listen on.

With this information it can tune in to that precise frequency an pick up the satellite a lot quicker with a lot weaker signal (such as when inside a building).

What I understand about A-GPS is that the device will use WiFi or cell phone signals when satellite signal is weak for faster lock-on and positioning, hence the A for Assisted. The iPhone has a true GPS chip built in.

I also tested above 2500 meters in South Italy where no cell signals was available and I wasn't able to have a position with GPS Kit and GPSlite on iPhone 3G. When I returned back to 2000 meters and below GPS worked fine again.

So I'm still not sure if GPS works without cell coverage?

GPS and Maps need a connection to the Internet to get the Maps. No cell or wifi, you are out of luck and off the map.

Exactly. At last a voice of reason on this forum rather than people guessing at answers.

Why did I have to switch to another provider before the app gets a GPS signal. I'm only telling my story.

it's rare to see so much misinformation packed into a single post.

I hardly know where to begin.

The best bet is to let that post stand on it's own merrit.

It has a real gps and will work where there is no cell service.

Any statement to the contrary is just wrong!

Go outside.

The GPS chip requires a clear view of the sky. Inside it relies on cell tower information to come up with an approximation.